The Invention concerns a method of operating an electrochromic element which consists of at least the following layers:
a first electrode layer;
first layer, in which ions can be reversibly inserted;
a transparent ion-conducting layer;
a second layer, in which ions can be reversibly inserted; and
a second electrode layer,
where the first and/or the second layer, in which ions can be reversibly inserted, is an electrochromic layer and the other of these layers acts as counter-electrode to the electrochromic layer, and where a voltage is applied to the electrode layers which induces a colour-change process, which voltage possesses values in a redox-stability range of the electrochromic layer system and the current flows through electrochromic element is measured continuously.
1. Field of the Invention
The term colour-change process denotes either forced colouring, that is to say a reduction in transmittance or reflectance of the electrochromic element, in particular in the visible region of the spectrum, or decolouring or bleaching, that is to say increasing the transmittance or reflectance. It can also however consist primarily of a change in the colour location of the transmitted or reflected radiation. Voltage values in a redox-stability range of the electrochromic layer system denotes voltages where the electrochromic layer system consisting of the electrochromic layer, the ion-conductive layer and the layer acting as counter-electrode experiences no or at all events very slight irreversible changes.
The electrochromic element incorporates at least one electrochromic layer, whose colour can be reversibly changed. This is combined as counter-electrode either with another electrochromic layer or with a transparent ion storage layer, which does not change its transparency significantly as a result of the insertion of ions. For the sake of simplicity, the two layers in which ions can be inserted are both designated below as electrochromic layers.
The layers of the electrochromic element mentioned above can also if necessary follow one another with further layers being interposed, such as for example protective layers, insulating layers, optically effective auxiliary layers, reference electrode layers, or the like. At least one of the electrode layers is a transparent layer. If the electrochromic element is to be used as a transparent window element with variable transmittance, the second electrode layer will also be transparent. If, on the other hand, the electrochromic element is to be used as a mirror with variable reflectance, one of the two electrode layers will preferably take the form of an opaque reflection layer of a suitable metal, such as aluminium or silver. It is also possible however to operate with two transparent electrode layers and to provide an additional metal reflection layer. For the sake of simplicity, only electrochromic elements with variable transmittance will be discussed, without however the Invention being restricted to this.
It is possible, by means of the voltage applied via the electrode layers to the electrochromic element, to alter its transmittance. This change generally takes place more quickly, the higher is the voltage applied. Of course, if the electrochromic element is not operated in optimum fashion, if therefore, in particular, the voltage applied is too high, it can be permanently damaged. It is then possible for the transmittance of the electrochromic element to cease being variable, or that the difference between minimum and maximum transmittance will no longer be as great as in undamaged state, under otherwise identical ambient conditions. It is also to be feared that the electrochromic element will no longer colour homogeneously, possibly irreversibly coloured or no longer colourable areas will be formed.
Above all, if a polymer electrolyte is used as ion-conductive layer, there is also a risk of the electrochromic layer delaminating, that is to say that the ion-conductive layer will become detached from the electrochromic layers in some areas.
According to the application of the electrochromic element, it will be exposed to a greater or lesser degree to wide temperature fluctuations. Thus, for example, in the case of an electrochromic element which is used in motor vehicles as window glass, roof glazing panel, or the like, it can be expected that it will operate satisfactorily at temperatures in the range of xe2x88x9220xc2x0 C. to +80xc2x0 C. Similar temperatures are to be expected in the case of applications in the outer skin of buildings, for example in the field of building curtain walls. It is known that a temperature increase will lead to reduction of the specific resistance of the system components. In particular, the resistance of the ion-conductive layer can decrease drastically with a temperature increase. If suitable measures are not taken, this can easily lead to the fact that, at high temperatures, the redox stability range of the electrochromic layer system will be exceeded and irreversible changes will occur.
2. Description of the Prior Art
From EP 0 475 847 B1, according to which the Preamble of the Patent Claim is formulated, a process for operating an electrochromic element is known, where the voltage applied to the electrochromic element is temperature-dependent. The temperature is measured directly with a thermometer, or indirectly, by a voltage pulse being generated prior to each colour-change process, by means of which with simultaneous current measurement, the resistance of the ion-conductive layer is determined, and from this the temperature of the electrochromic element is determined. According to the temperature determined, a voltage is applied to the electrochromic element for a predetermined time. When the desired transmittance is reached, the voltage is disconnected.
EP 0 718 667 A1 has as its subject a process for operating an electrochromic element which can be influenced by the user, which process can be adapted via an interface to electrochromic elements of different designs, to the ambient temperature and to the dimensions of the electrochromic element. Here, the voltage with which the electrochromic element is operated is also to be a function of the temperature. A disadvantage of the known process is that, for each individual electrochromic element, matching of the control parameters to the window dimensions must take place.
EP 0 683 419 A1 discloses a method to trigger an electrochromic element in which a current is impressed on this.
The purpose of the present Invention is to provide a process for operating an electrochromic element which will operate over a wide temperature range, which is largely independent of the area of the electrochromic element, which permits a change in transmittance over a wide range, which permits sufficiently rapid colour change, and with which a long service life of the electrochromic element can be achieved.
This problem is solved by a process in accordance with Patent claim 1. Advantageous configurations are the subject of the Subclaims.
According to the Invention, provision is made for the current I flowing through the electrochromic element to be measured continuously, for the voltage U applied to the electrochromic element during a starting stage of the colour-change process to be increased or reduced continuously up to maximum to a final value Umax predetermined as a function of temperature, where the temperature dependence of the final value Umax is determined by the design of the electrochromic element, but is independent of the area to be subjected to colour change, and that the voltage U is controlled during the course of the colour-change process as a function of the current I, where the voltage U does not exceed in magnitude the magnitude of the final value Umax. The final value Umax can possess a different magnitude for a colouring process than for a bleaching process.
Current measurements will normally take place regularly at always the same, sufficiently short intervals of time, typically several times a second. It is also possible to proceed in such a way that, for example in the starting stage of the colour-change process, measurements are carried out at shorter intervals than in later stages, because in the starting stage, the current and the voltage will normally change at the fastest rate.
For most applications, it will suffice for the temperature dependence of the final value Umax of the voltage U is determined by a linear relationship, for example:
xe2x80x83Umax=Axe2x88x92Bxc2x7T,xe2x80x83xe2x80x83(1)
where T is the temperature of the electrochromic element, and A and B are constants determined by the design of the electrochromic element, which are to be established empirically. If the temperature T is in xc2x0 C., A will correspond in value to the voltage U which may be applied as maximum to the electrochromic element at 0xc2x0 C. With the constant B is determined to what extent the final value Umax of voltage U is to be modified in the event of temperature changes. A and B may be different for a colouring process and a bleaching process respectively. They are characteristic of a certain design of electrochromic element, but independent of its dimensions. They can be established on the basis of cyclic voltammetric preliminary investigations on the electrochromic layers, for example by means of systematic test series, where electrochromic elements of the same design and dimensions are operated with different values of Umax at different temperatures, and the maximum voltages in magnitude determined at which the electrochromic elements do not, over a large number of (minimum approximately 1,000xe2x88x9210,000) colour-change cycles, experience any significant deterioration in their colour-change properties.
An essential feature of the Invention is the regulation of the voltage U applied to the electrochromic element as a function of the current I. The Invention utilizes the surprising principle that evaluation of the continuous measurements of the current I renders superfluous any knowledge of the exact dimensions of the electrochromic element for safe operation of this element. The measured values of the current I can basically be utilized in a different fashion for regulation of the voltage U. Thus, for example, provision can be made for the voltage U to be increased in magnitude initially in the starting stage up to the final value Umax and subsequently to maintain it at the value reached until the current I falls below a temperature-dependent first threshold referred to the maximum current Imax xe2x80x94as explained in detail belowxe2x80x94following which the voltage U is reduced in value continuously or in several steps, until the current I reaches a lower switch-off threshold also referred to the maximum current Imax, dependent on temperature. Advantageously, this current-controlled regulation of the voltage U takes place however with the aid of an arithmetic value for the total resistance Rges of the electrochromic element determined from current and voltage measurements.
The total resistance Rges of the electrochromic element can be determined preferably in the starting stage of the colour-change process from the voltage U and the current I. To compensate for any voltage offset (open-circuit voltages), the total resistance Rges is preferably calculated as the first derivative of the voltage U to the current I. This is obtained in first approximation by the formation of the quotient xcex94U/xcex94I of the magnitudes of voltage difference and current difference at consecutive moments of time ti, ti+1, xcex94U=|U (ti+1)xe2x88x92U (ti)|, xcex94I=|I (ti+1)xe2x88x92I (ti)|. The accuracy of the calculation can be increased by averaging being carried out from several quotients xcex94U/xcex94I determined at different points in time. By carrying out the measurements and calculation in the starting stage of the colour-change process, it is possible to a large extent to avoid falsifying the measurement results due to internal voltages occurring during the course of the colour change.
As the total resistance Rges is temperature-dependent, it is basically possible to conclude the temperature T of the electrochromic element from this, if it should be necessary to dispense with separate temperature sensors. Particularly in the case of large-area electrochromic elements, preference should of course be given to direct temperature measurement with the aid of a temperature sensor, on account of the greater degree of accuracy obtained.
Especially long service life of the electrochromic element can be achieved by calculating from the voltage U, the current I and the total resistance Rges, a voltage Ueff which is effective electrochemically at the electrochromic layers, and by regulating the voltage U such that Ueff does not in magnitude exceed a predetermined value Ueff,max, above which irreversible changes can occur at the electrochromic element. Here, the following Approximation Equation is preferably used to determine the voltage Ueff effective electrochemically at the electrochromic layers; how it is arrived at is described below:
Ueff=Uxe2x88x92Ixc2x7Dxc2x7Rgesxe2x80x83xe2x80x83(2)
where D is a correcting variable, to be used where necessary to compensate for approximation errors. It will suffice in most cases to use the value of 1 for D. To optimize the voltage regulation with a view to maximum possible service life of the electrochromic element, it may however be advantageous to work with a correcting variable D differing from 1, this being determined in orientation trials. Cases are conceivable, for example, where measurement of the total resistance Rges is only carried out at a relatively late stage of a colour-change process, in which the individual resistances of the electrochromic layer system depend to an especially large extent on the colouration state or in which the electrochromic layers possess an unusually high ohmic resistance.
Rapid, but nevertheless careful colour change is achieved by an especially simple method of control if, after completion of the starting stage, and as long as the voltage Ueff electrochemically effective at the electrochromic layers does not yet reach the maximum permissible value Ueff,max in magnitude, the voltage U is kept essentially constant at the final value Umax, reached at the end of starting stage. Of course, it could even be possible to operate with voltages U which are lower in magnitude than the final value Umax. Such a process would however result in longer colour-change times, which is normally undesirable.
A switch-off criterion of the voltage U can, in particular where complete colouring or bleaching is desired, be defined particularly simply according to the Invention with the aid of the maximum current Imax which has flowed during the colour-change process. It can thus be determined that the voltage U is switched off when the ratio of instantaneously flowing current I to maximum current Imax falls below a specified value which is determined by the design, the type of colour-change process and generally by the temperature T.
If only partial colour change is desired, it is possible for example to monitor the transmittance or the reflectance of the electrochromic element and to switch off the voltage U when the transmittance or reflectance reaches a predetermined value.
Another alternative consists of determining the quantity of electricity which has flowed in the electrochromic element since commencement of the colour-change process and to switch off the voltage U when the quantity of electricity which has flowed reaches a specified value. The quantity of electricity which has flowed can be determined by time integration of the current I.
With the process according to the Invention it is possible, as soon as the design-related parameters A, B, D, Ueff,max and the switch-off ratio I/Imax have been determined, to carry out self-calibration of the control process, essentially independently of the area of the electrochromic element to be subjected to colour change, which will permit safe operation of the electrochromic element. It lies within the scope of the Invention however, for the purpose of refining and further optimizing the control process to define various size classes for electrochromic elements, within which in each case the same design-dependent parameters are applied, for example in increments of approximately 0.5 to 1 meter, referred to the shortest element dimension.
The process according to the Invention permits, with a simple method of control, rapid, reproducible and uniform colour change of electrochromic elements, where additional switch-off criteria can be applied for partial colour change. In practical form, it is determined decisively by its starting stage, in which self-calibration is carried out, that is to say in which essential control parameters of the process are determined.